1. Field of the Invention
The present invention relates to a humidifier for humidifying dry gas by transferring moisture from moist gas.
2. Description of the Related Art
Recently, fuel cell electric vehicle (FCEV) has come to attention mainly from the viewpoint of suppressing carbon dioxide emission which is one contributor to global warming. In such a fuel cell electric vehicle, a fuel cell (FC) generates electricity by means of an electrochemical reaction of hydrogen (H2) and oxygen (O2) contained in air, and the generated electricity is supplied to a motor which in turn generates drive power.
With respect to the fuel cell for FCEV, a fuel cell of PEM (proton exchange membrane) type, for example one in which solid polymer is used as an electrolyte, has been widely used since the solid polymer endows the fuel cell with advantages, such as small size, light weight, high efficiency and high output. Among the solid polymers is fluorocarbon polymer used in the form of an ion-exchange film. It functions as a proton conducting electrolyte when containing moisture. However, when moisture is not present, its proton conducting property is greatly lost and electrical contact with the electrodes becomes poor, resulting in drastic drop in the output. In order to overcome this deficiency of the fuel cell system, it is necessary that the gases supplied to the anode and the cathode (i.e. anode gas and cathode gas) be humidified using a humidifier so that a certain level of humidity is maintained in the electrolyte.
As for the humidifier to be used for humidifying a reaction gas, various types have been proposed and used. Japanese Unexamined Patent Application Publication 2003-157872 (see paragraphs [0014] and [0020] and FIG. 1) proposed a humidifier in which a reaction gas is humidified through hollow fiber membranes utilizing moisture contained in off-gas exhausted from the fuel cell, since no external water supply nor electric power is required, and in addition, humidification requires no complex structure. One example of this type of humidifier is mainly composed of a humidifying module including a bundle of a plurality of hollow fiber membranes contained in a cylindrical case, and heads attached to the both ends of the humidifying module. A reaction gas (hydrogen gas or oxygen-containing gas) is introduced into the humidifying module from a feed inlet formed on one of the two heads, and after flowing along the hollow part of the hollow fiber membranes, it is exhausted from an outlet formed on the other head. On the other hand, off-gas (anode gas or cathode gas) is introduced from an inlet formed on the latter head into the humidifying module, and after flowing outside the hollow fiber membranes, it is exhausted from an outlet formed on the former head. With these flows of the gases, moisture contained in the off-gas permeates the pores on the hollow fiber membranes due to the difference in partial pressures of water vapor across the membrane, and humidifies the reaction gas running along the hollow part of the hollow fiber membranes.
With such a humidifier using the hollow fiber membrane, it is inevitable that moisture in the reaction gas will be condensed into water when the reaction gas humidified in the humidifying module is brought into contact with the head with lower temperature than that of the reaction gas. If the condensed water remains in the head during nonoperational period of the vehicle, problem arises in that, for example, during severe cold weather, the condensed water may become frozen and clogs the flow passage of the reaction gas, leading to low performance of the fuel cell system upon the next operation. In order to solve this problem, various proposals have been made, in which the condensed water is expelled through drain piping to the external system (e.g. beneath the vehicle), or in which the condensed water is collected in a drain tank mounted on the rear part or the like of the vehicle. However with these proposals, there still remains a problem in that the condensed water is not drained from the head at the next operation, if the condensed water in the drain piping or drain tank became frozen during nonoperational period of the fuel cell system.
The present invention has been made based on the above-mentioned background, and the object is to provide a humidifier in which the clogging of the gas flow passage caused by the remaining condensed water is prevented, while keeping the humidifier still simple and cost-effective to manufacture.